The present invention relates to implantable electrical leads generally, and more particularly, to implantable cardiac pacing leads.
Implantable cardiac pacing leads and other medical leads for permanent implant typically employ either cabled conductors or coiled conductors, coupling an electrode or other electrical component located on the lead body to a connector assembly at the proximal end of the lead body. As the designs of implantable electrical leads have progressed over the years, there has been a general trend toward reduction in the diameter of the body of such leads, with further reductions in lead body diameter to be desired. However, as the diameter of the lead body is reduced, producing a lead having an adequate tensile strength becomes correspondingly more difficult.
One approach to providing a small diameter lead having a high tensile strength is to fabricate the lead using an inextensible conductor, for example a stranded conductor as disclosed in U.S. Pat. No. 5,246,014 issued to Williams et al., a cabled conductor as disclosed in U.S. Pat. No. 5,584,873 issued to Shoberg et al, or a tinsel-wire conductor as disclosed in U.S. Pat. No. 3,844,292 issued to Bolduc, all incorporated herein by reference in their entireties. One approach to increasing the tensile strength of a lead including a coiled, normally extensible conductor, is to provide a reinforcing fiber or core within the lead, as disclosed in U.S. Pat. No. 5,231,996 issued to Bardy, et al and U.S. Pat. No. 5,056,516 issued to Spehr, both also incorporated herein by reference in their entireties. As a practical matter, however, the designs revealed in the disclosed Spehr and Bardy patents are difficult to implement in the context of a lead having a diameter of less than 3 or 4 French.
The present invention is directed toward an implantable electrical lead having a high tensile strength and having a lead diameter less than about 4 French, preferably less than about 3 French, more preferably about 2 French. A lead according to the present invention meets the desired criteria by incorporating a helical, coiled conductor that is fabricated by winding the conductor around a twisted fiber core. In a preferred embodiment, the fiber core takes the form of two lengths of fiber cord twisted to provide a core having a generally circular cross-section, around which a single or multi-filar coil is wound. The coil is preferably wound tightly enough to compress the fiber core slightly, and more preferably wound tightly enough to compress the fiber core approximately 20% to provide a composite coil/core structure having high flexibility and high tensile strength. In preferred embodiments of the invention, the composite coil/core structure is coupled to a connector assembly at its proximal end and an electrode or other electrical component located on a more distal portion of the lead body. The interconnections of the composite coil/core structure with the electrical components of the lead are preferably configured such that the mechanical interconnections of the fiber core with the electrical components are independent of the electrical connections between the coil and the electrical components, protecting the coil and outer insulation from damage due to tensile forces applied to the lead body.
In preferred embodiments of the invention, the fiber core may be produced by folding an elongated cord back on itself to produce two co-extensive lengths of cord and twisting the ends of the cord to provide a structure having a closed loop at one end. This closed loop may be employed to couple the fiber core mechanically to one of the electrical components of the lead. In a preferred embodiment, the loop is located at the distal end of the lead and is employed to mechanically connect the fiber core to an electrode.
An additional mechanism for interconnecting the fiber core with an electrical component of the lead is to provide a stepped lumen in the component, tie the fiber core into a knot and locate the knot in a wider diameter portion of the stepped lumen to mechanically couple the fiber core to the electrical component. In one preferred embodiment a knot is employed to couple the fiber core to a tubular connector member located at the proximal end of the lead, and the fiber core is allowed to extend proximally from the connector member to facilitate handling of the lead. For example, the proximally extending fiber core may be employed to thread the proximal end of the lead into an adaptor as illustrated in U.S. Pat. No. 5,246,014 and may be snipped off thereafter. Alternatively, the portion of the fiber core extending proximally from the connector assembly may be snipped off prior to insertion of the connector into the connector block of an associated implantable medical device such as a pacemaker or other stimulator.
In one preferred embodiment, the tubular connector member is mounted over the fiber core prior to winding of the conductor coil, and the conductor coil is wound around both the fiber core and the tubular connector member to provide an electrical connection between the coil and the connector member. In this embodiment, it is preferable that the tubular connector member be provided with a distally directed extension along one side of the distal end thereof to facilitate the winding of the conductor at the point of transition from the fiber core to the tubular connector member.